This invention relates to traction control of railroad locomotives; and more particularly, to a system and method of enhancing locomotive adhesion control using creep and adhesion measurements of all the axles, and the proximity of an axle to each of the other axles to affect the adhesion of each individual axle.
Railroad locomotives must provide a great degree of traction under a wide range of rail conditions; i.e., dry, wet, icy, oily. Generating the maximum tractive effort of a locomotive, or a consist of locomotives, produces the most efficient and effective operation of the train. Developing the maximum tractive effort by a locomotive requires that each axle of the locomotive, which includes the traction motor and wheels associated with the axle, develops its maximum tractive effort.
In a moving train, developing the maximum tractive effort by each axle is a dynamic function dependent upon a number of factors some of which can be controlled, and some of which cannot. Among the latter are rail conditions. It will be appreciated by those skilled in the art that tractive effort is limited by the amount of contact friction between the wheels of the locomotive and the patch of rail over which the wheels are passing at any given moment. This amount of friction, in turn, depends such factors as the presence of contaminants (oil, or lubricants such as sand) on the rail or wheel, the shape (roundness) of the wheel, the shape of the rail, atmospheric temperature, and the normal force or weight imposed on an axle, among others.
Referring to FIG. 1, an exemplary railroad locomotive V has a forward truck or bogey K1, and a rearward truck K2. Each truck has multiple axles. In FIG. 1, three axles are shown with truck K1 having axles A1–A3, and truck K2, axles A4–A6. Wheels W are mounted on each end of each axle. The locomotive travels over a set of rails indicated generally R. In many locomotive configurations, the locomotive's wheels are driven by electric traction motors, as is well-known in the art. This allows for torque control to be separately established for each locomotive, for each set of axles, on a per axle basis, or on a per truck basis. Modern adhesion control systems attempt to maximize the tractive effort delivered to the rail by controlling the creep of the wheels through the amount of torque applied to the axles.
Creep is defined as follows:
      Creep    =                              wheel                ⁢                                  ⁢                  (          W          )                ⁢                                  ⁢                  speed                    -              train  speed                    train  speed      
In U.S. Pat. No. 6,163,121 which is assigned to the same assignee as the present application, there is described a method and traction control system for a locomotive which separately controls the allowable creep level of each axle; i.e., the axles A1–A6 in FIG. 1. In the control system described therein, the tractive effort generated by each axle (including its associated traction motor and wheels) is monitored. Control signals are then generated and supplied to the traction motor for the axle to produce the amount of creep necessary to achieve the maximum tractive effort.
A problem with current control systems is their response time to a change in road conditions. This time can be in excess of ten seconds between a change in rail conditions and the resulting system response to change traction motor operation to produce the maximum tractive effort for these new conditions. Accordingly, in a moving train, rail conditions may change significantly between a change in conditions is sensed and the system reacts to produce the maximum tractive effort for previous rail conditions.
Regardless of how torque control is applied; i.e., on a per axle, per set of axles, or per locomotive basis, adhesion control systems typically measure, directly or indirectly, the speed of each wheel together with the speed of the locomotive. Wheel speed and mathematical derivatives of wheel speed are then used, together with the measured or calculated locomotive speed, to adjust the amount of torque applied.
Referring to FIG. 2, adhesion is determined by the equation:
      Adhesion    =            tractive  effort                      Weight  of  locomotive            ⁢                          ⁢      V      
In FIG. 2, separate performance curves are presented for a variety of different rail conditions including a dry rail, a dry rail with sand on it, a wet rail, and a rail with oil on it. These curves are illustrative only, and those skilled in the art will understand that the actual relationship between friction and creep may be different. The respective curves are a measure of adhesion with respect to per unit creep for each of the different conditions. Peak points a, b, and c are indicated on the curves for a dry rail with sand, a dry rail, and a wet rail respectively. If a locomotive has individual axle torque control, as taught by the U.S. Pat. No. 6,163,121, the optimal creep level is separately controlled for each axle.
FIG. 3 is a simplified block diagram illustrating a prior art individual axle adhesion control system. In this system, a wheel creep controller WCC dynamically adjusts the amount of torque applied to an axle, with wheel creep being limited to a value established by a tractive effort maximizer TEM. Maximizer TEM dynamically adjusts the creep limit output value supplied to controller WCC, so to attain and maintain the peak values (a, b, c) for the respective adhesion curves shown in FIG. 2. Controller WCC, in turn, supplies a creep torque limit output to a traction motor torque controller TMTC, whose output drives a traction motor TM for the individual axle.
Axles A1–A6 on locomotive V travel over the rails R in a sequential fashion. The condition of rail R and the adhesion curves such as those in FIG. 2 vary from axle to axle for a number of reasons. These include:                a) rail cleaning due to wheel/rail contact patch interaction;        b) sand or friction enhancer applications to the rail;        c) wayside, on-board, rail, or flange lube applications;        d) differences in the normal force (including weight) on an axle; and,        e) contact patch and trajectory changes (since all of the axles may not be traveling exactly over the same path on the rail all of the time)        
FIG. 4 illustrates the adhesion of three sequential axles moving over a rail. In FIG. 4, the plots assume that there are no substantial differences in friction between the axles. FIG. 5 is an enlarged version of a portion of the plots in FIG. 4. In FIG. 5, the points indicated L, M, and T represent the axle creep for a respective leading axle L (A1 or A4), middle axle M (A2 or A5), and trailing axle T (A3, A6) on a truck (K1, K2). As shown in the Figure, leading and trailing axles L and T are not operating at their peak or optimal creep levels, while middle axle M is operating at its peak, optimal creep level. If various factors such as rail cleaning and normal force differences between the axles are negligible, then the creep value for the axle (axle M) producing a substantially higher tractive effort than the other two axles on the truck provides a goal target for the creep value the other two axles on the truck should attain.
FIG. 6 illustrates how creep limit values may be adjusted for individual axles to increase their respective tractive efforts. The present invention is directed to augment the adhesion control system shown in FIG. 3, and described in the U.S. Pat. No. 6,163,121. As described hereinafter, control information such as that shown in FIG. 6 is combined with the individual axle information; e.g., measured slope of an adhesion curve (ΔTE/Δcreep) for the particular axle, to couple all of the locomotive's axles together to improve the overall tractive effort of locomotive V.